Yes, 30mA is a little high for personnel protection. I think it's because the Regs. are still very much geared around the idea of a single GFI protecting the whole installation (or at least a section of it) rather than an individual branch circuit. If you go below 30mA on a whole-house GFI you start getting a lot of problems with nuisance tripping, hence the 30mA specification. Until the last few years, the main GFI was more often 100mA.
Originally, buildings with a direct ground connection via the cable or on PME (ground bonded to neutral) had no main GFI. But most rural areas just grounded the building to a local rod, and the resulting high earth-loop impedance meant that the breaker was needed as even a 5A lighting fuse couldn't clear a ground fault. (Early residential were the voltage type ELCB, as in my diagrams.)
More recently, the Regs. changed to specify a maximum disconnect time: 5 secs. for a fixed appliance, wall lights, etc. and 0.4 secs. for outlets feeding portable appliances. If normal fuses or MCBs can't clear a ground-fault in that time, then a GFI is needed. In practice, this still means that in most cases buildings on PME or solid-ground don't need one but ones grounded to a local rod do.
We can get GFIs for individual circuits, and receptacles incorporating more sensitive GFIs have started to become available, but they've yet to become widespread like the 6mA (?) branch GFI in the U.S.
